This invention relates to power amplifier control circuits. More specifically, the invention relates to saturation detection and control for power amplifiers.
The use of power amplifiers in transmitting radio frequency (RF) signals has many applications, including, but not limited to radiotelephone communications systems. A typical radiotelephone communications system includes multiple fixed site transceivers. Each fixed site transceiver is an interface between the line telephone system and multiple portable, or mobile radiotelephone units located within a geographic area served by the fixed site transceiver. The fixed site transceiver and the radiotelephones communicate by sending radio frequency (RF) signals to each other.
Radiotelephones are generally of two different types. Some conventional radiotelephone systems employ analog units that are basically the equivalent of a walkie-talkie. Each analog unit communicates voice messages by broadcasting a radio frequency (RF) carrier signal which has been modulated in some fashion by an analog signal corresponding to the voice message. Other radiotelephone systems employ a digital unit. Digital units convert the speech into a digital representation and then broadcast a radio frequency (RF) carrier modulated with the digital representation of the speech.
Analog radiotelephone systems typically employ a limited RF spectrum for radiotelephone communications. According to one conventional communication method, the RF spectrum is divided into relatively narrow segments of frequency. Upon request, each radiotelephone is allotted one of these dedicated segments in which to broadcast and receive signals from the fixed site transceiver. This method of communication is known as Frequency Division Multiple Access (FDMA). Using this method the radiotelephone transmitter would turn on and remain on the fixed frequency for the duration of the call. If the turn on functions and turn off of the transmitter are limited to the beginning and end of the phone call, the turn on and turn off function""s timing requirements are not very stringent.
There are several difficulties with the described FDMA system however. One of the difficulties is that, in portable units, keeping the transmitter powered during the course of a telephone call can consume a significant amount of energy. Since operation time is limited by the amount of energy contained within the portable unit, it is typically desirable to minimize power consumption and thereby increase the portable unit""s operating time.
Another problem with the analog version of the FDMA system is that, because it is an analog system, it is prone to the usual problems inherent in analog systems such as spurious signals, interferences from other sources of RF energy, multipath reception, and fade outs. The same types of problems will occur with digital systems, but because they are digital, error correction coding and a variety of other digital and software techniques help compensate for these difficulties. Digital systems can help to more efficiency use precious bandwidth providing more users than analog systems for a given level of quality. Accordingly, alternate methods of communication have been developed such as Time Division Multiple Access (TDMA). This method operates by sharing a single frequency band among users by dividing the band into time slots and allotting a time slot to each radiotelephone unit. Each radiotelephone unit then broadcasts data during its allotted time slot and stops transmission until the next time allotted time slot occurs, and then the radiotelephone unit broadcasts again. This method has advantages which address many of these aforementioned analog FDMA problems.
First, because the radiotelephone is actually broadcasting only during it""s own time slot, there is a reduction in the power consumed because there is no need to keep the RF power amplifier of the transmitter on continuously during the call. In fact if the RF power amplifier of the transmitter did remain on during the entire call it may result in interference with other units using successive time slots. The RF power amplifier in a mobile radiotelephone usually requires a relatively large amount of energy and is therefore a significant contributor to battery drain. Because in TDMA systems the power amplifier of the transmitter is actually turned off most of the time a significant saving in terms of energy consumption can be realized.
Second because continuous speech is being transmitted, and only time slots are available for broadcast, it is convenient to represent TDMA signals in digital format. The speech must be encoded into discrete portions to fit in time slots in such a way that continuous speech can be recreated at the receiving end. Because TDMA is digital, further techniques such as digital data compression and various digital coding techniques may be used to minimize transmission errors.
The use of TDMA, however, can bring a new set of constraints. One of these constraints involve the requirement of transmitter control for limiting transmissions to the allotted time slots only. This type of transmission, often called burst or pulse transmission mode, involves turning on the RF power amplifier just after the beginning of a time slot allotted to the radiotelephone unit, increasing the power to a predetermined level, transmitting the encoded signals during the time slot, decreasing the power, and finally shutting off the RF power amplifier near the end of the time slot. One of the problems inherent in such a mode of transmission is the possibility of spurious RF radiation that can be created if the RF power amplifier, or any solid state device, is turned on or shut off too quickly, or in a non linear fashion.
A second problem is the tendency for a pulsed radio to disturb neighboring frequency channels, a phenomenon often called AM splash. AM splash generally occurs when a power amplifier ramps up too fast causing energy to appear outside the allotted transmission bandwidth.
A third problem can occur when an RF power amplifier does not turn off prior to the end of the allotted time slot thereby disturbing the transmission on the succeeding time slot. All these problems are heightened by the fact that a pulsed transmission system turns on and off many times per second and so any interference generated tends to happen repeatedly and continuously during the time the interfering system is activated.
To combat these problems standards have been developed regarding burst mode radiotelephone transmission. On of the most popular standards is the Global System for Mobile Communications (GSM) format. The GSM format is the basis for the European Personal Communications Standard (PCS) and has also found wide acceptance in North America as the PCS-1900 standard. In order to insure that the aforementioned problems are minimized the GSM format includes a power versus time templates that specify power limits for broadcasting in a burst mode in such a way as to minimize unwanted interference. This template specifies the desired power output level ranges versus time for a TDMA time slot. The GSM power template dictates a maximum and minimum power level for each point on the curve. It is desirable to limit the power output to values inside the GSM power template, to control the desired level for transmission. It is also desirable to have a smooth turn on and turn off of the RF amplifier, to minimize the possibility of spurious RF generation. Many radiotelephone systems implement the GSM standard by controlling the RF power amplifier in such a manner that it remains within this template.
In an analog control system a comparator compares a desired level of a preset variable, called a setpoint or reference signal, with the value of the variable the control system is attempting to control. The comparator generates an error signal which represents the difference between the desired setpoint, and the actual value of the variable that the control system is attempting to control. This error signal is then used as a control signal to adjust the system to minimize the difference between the setpoint and the actual value of the controlled variable. Generally a setpoint represents the desired value of a variable. A popular method of following the GSM power versus time template is to use an ordinary analog control system and control the setpoint to be a point within the GSM template.
In the case of TDMA radiotelephones utilizing the GSM power template the setpoint represents the desired output power from the RF power amplifier of the radiotelephone. The analog control system receives as its setpoint input a point within the GSM power template. The control system compares the setpoint, with the power output from the radiotelephone""s RF power amplifier (the controlled variable) and generates an error signal. This error signal corresponds to the difference between the setpoint and the actual power output of the radiotelephone. This error signal is then used to adjust the RF power amplifier. If the power from the RF power amplifier is too low the error signal will be used to increase the power output from the RF power amplifier, which in turn will reduce the error signal. This process will continue until the RF power amplifier""s power output matches the desired value. At that point the error signal will indicate that no further adjustment is necessary. Similarly if the power from the RF power amplifier is too high the error signal will be used to decrease the power output from the RF power amplifier, which in turn will reduce the error signal. This process will continue until the RF power amplifier""s power output matches the desired value. At that point the error signal will indicate that no further adjustment is necessary. This method of burst control transmission can, however, present problems as the batteries powering the unit run down. The amount of power that any power amplifier, including RF power amplifiers, can deliver is a function of it""s battery voltage and other factors such as temperature. As batteries are diminished, the voltage level provided by the batteries to the amplifier diminish, which at some point can result in diminished amplifier output power. If the amplifier output drops below the setpoint the amplifier control system comparator will provide a control signal to the amplifier attempting to increase the power output of the amplifier as described above. However, because the battery voltage input to the amplifier has diminished, the amplifier may not be able to increase it""s output power. Thus when this point is reached increasing the input to the power amplifier or attempting to increase the output of the power amplifier will have no effect. At this point the power amplifier control loop is said to be saturated, in other words it has reached it""s maximum.
At or near the saturation point, some of the properties of the power amplifier, such as its ability to reproduce signals accurately, linearly, and without distortion may be reduced. Distortion may cause spurious RF interference to be created, which may interfere with other radiotelephones as well as other circuitry within the radiotelephone. Saturation of the power amplifier may also produce unwanted side effects such as a reduction in efficiency of the amplifier and overheating.
Another issue related to the fact that the power amplifier cannot deliver as much power as the control circuitry setpoint demands is the loss of control of the power amplifier by the control circuitry. If the control circuitry detects that the power output of the power amplifier is less than the desired setpoint, an error signal will be created. This error signal will result in an attempt to increase the output of the power amplifier. If the power amplifier is saturated the power will not increase. If the power output does not increase, the error signal will increase as it tries further to correct the difference between the desired and actual power output of the system. Even though the error signal increases the saturated power amplifier can deliver no more power. The error signal will, however, continue to increase as the control system continues, without success, to try to increase the power out of the power amplifier. The control system error signal will be maximized, if the setpoint is not reduced. The consequence is that the error signal will increase from the point at which saturation occurred to its maximum value. Throughout this range (RSAT) the changing of the error signal has no effect on the power output. Then when the control system tries to shut the power amplifier off, such as at the end of its burst transmission time slot, it will do so by lowering the desired setpoint. The lowering of the setpoint will lower the error signal, but this will have no effect if the error signal is within the RSAT range. The control system will continue trying to decrease the power amplifier""s power output but this will have no effect until the error signal has decreased below the lower end of the RSAT range. When the error signal finally does get below the RSAT range the power amplifier may be out of the desired power template and significantly off the setpoint. This discrepancy between the desired power setpoint and the actual power may increase the speed at which the power amplifier shuts off. This in turn may cause spurious RF radiation if the turn off function is too sudden. In addition because the power may not be decreasing according to the GSM power template recommendations, the actual output power before the amplifier is turned off may run over into the adjacent time slot, thereby interfering with a transmission occurring in that succeeding time slot. In addition this less than ideal turn off may cause excess power to be dissipated and result in the generation of excess heat. These problems are increased by the fact that, because the burst transmission mode is repetitive, these turnoffs occur over and over several times per second and any spurious RF radiation is repeatedly generated, and excess heat generation will tend to accumulate.
Accordingly, preferred embodiments of the present invention relate to communication systems and process and components thereof which address the above noted problems associated with TDMA transmissions within desired power profiles.
According to one embodiment of the present invention a transmitter has an amplifier, an amplifier controller, and a digital signal processor wherein the controller has a comparator with a reference input controlled by two inputs. The first input is generated by a digital signal processor, and the second input is generated by a power output sensor connected to the output of the amplifier. A current sensor is provided for determining the current flowing to the amplifier from the amplifier""s power source. The output of this sensor is also connected to the digital signal processor. The digital signal processor detects the current flowing to the amplifier. In one embodiment of the invention the digital signal processor compares the amount of current flowing to a known safe value and by cutting back the setpoint of the power amplifier whenever the current increases beyond the known safe value prevents the saturation of the power amplifier.
In another embodiment of the invention the digital signal processor compares the change in the power amplifier""s setpoint to the change in the current drawn by the power amplifier, and by detecting a proportionally diminishing current drawn by the amplifier detects the onset of saturation of the amplifier control system. When the saturation point is detected in this manner the digital signal processor can prevent saturation by limiting the setpoint of the power amplifier to values which are less than the value at which point the saturation is detected.